How To Execute Part Of WHERE Clause Conditionally

I need to insert Data into a table based on particular condition, So I'm using an Insert select as follows:

INSERT INTO error_table
(error_code,cust_id,customer_name,error_message )
SELECT '34', cust_id,customer_name, 'Missing xxx'
FROM customers
WHERE cust_id = cur_cust.cust.id
AND ( NOT EXISTS ( Select .....) -- control 1
OR (EXISTS ( Select .....) --control 2
OR (NOT EXISTS ( Select .....) -- control 3

I have around 10 controls. I want these controls to run conditionally..I was wondering if this can be done using a decode based on some variable outside the query.

I want the query to run like this:

if A then execute control 1: AND ( NOT EXISTS ( Select .....)

If B then execute control 2: AND ( OR EXISTS ( Select .....)